As Brazilian tribes gather at the World Social Forum to call for greater rights for indigenous peoples one group has accused US scientists of taking their blood to carry out research without their consent.

And the Yanomami people, who live in northern Brazil, are demanding that the US universities holding their blood return it.

The tribe accuses Napoleon Chagnon and James Neel, two US scientists who worked with Indians from the 1960's until the mid 1990's, of taking their blood without obtaining proper permission.

Davi Kopenawa, Yanomami leader "The anthropologists took advantage of us, because we did not know how to defend ourselves, or how to deny them from taking our blood," Davi Kopenawa, a Yanomami leader who had blood taken from him when he was a child, told Al Jazeera.

"They did not explain what was the purpose of the blood samples they were taking. What was our blood going to be used for, by the white man? Were they going to drink it?

"Or are they going to use it to experiment with other men, to make them Yanomami?"

One accusation that still reverberates today, is that Chagnon coerced the Yanomami into giving up blood samples by bribing them with gifts, including weapons such as machetes they would later use in conflicts among themselves.

Chagnon has also been accused of misleading the Yanomami into thinking the blood would be used in ways that would directly benefit them, when instead it would be used for scientific research.

'Mending fences'

In 2000, author Patrick Tierney published a book containing criticism's of the scientists work.

Darkness in El Dorado: How Scientists and Journalists Devastated the Amazon, contained page after page of accusations against Chagnon and Neel.

Tierney's book grabbed headlines all over the world, sparked at least half a dozen government investigations in Venezuela and Brazil - where the research took place - and turned the anthropology world upside down.

The book also alleges that the Neel and Chagnon teams exacerbated a measles epidemic during their fieldwork with the Yanomami by administering improper vaccines.

That claim is also widely disputed.

The book also sparked action. Yanomami leaders, most of whom were children when their blood was extracted, started a concerted effort to request the return of their blood in 2001.

They have been helped by sympathetic anthropologists and indigenous rights groups who saw the return of the blood as a first step to mending fences with the Yanomami people and putting to rest some of the damning accusations in Tierney's book.

Blood held

Most of the blood taken during the expeditions is held in the US by Kenneth Weiss, an anthropologist at Penn State university.

Weiss has never been to Brazil and has no particular research interest in the Yanomami people, but had inherited the samples from Neel, his former professor who was part of the original expedition in the 1960s.

Since 2001, when the controversy over how the samples were collected broke with the publishing of Tierney's book, he halted any research using the samples and now forbids any of his students to use them.

The other institutions that have significant Yanomami blood samples are the University of California at Irvine, Binghampton State University in New York, and the US government's National Cancer Institute.

Few involved have spoken about the case, other than to say they want to return the blood.

In an email statement, Gerald Sonnenfled, a Binghamton University representative, would only say they had agreed to return "all requested samples about a year ago and we still await instructions from the government of Brazil on how to do so".

The National Cancer Institute, part of the US government's National Institute of Health in Maryland, has specimens from about 475 individuals, said Karen Pratt, who oversees the Yanomami samples there.

Pratt told Al Jazeera the samples were used for one experiment in 1992 and have not been used since, adding the institute wants to return the blood, but is waiting to hear from the Brazilian government as to where to send the specimens.

Biohazard fears

Weiss also says he wants to return the samples to Brazil but that the move has been delayed because the blood is a potential biohazard, which dramatically complicates the shipping.

Furthermore, the blood has been frozen for so many years that once it is brought to room temperature, it could explode, spattering potentially hazardous blood over those around.

"This has been so contentious that if we send samples back and anything goes wrong, I'll get the blame even though I have had nothing to do with collecting them," Weiss told Al Jazeera.

"And so we certainly don't want to be accused of giving disease to the Indians by sending them back hazardous material."

The Brazilian prosecutor's office in the state of Roraima, where much of the blood was collected, has formally requested the blood be returned, however there appears to be few people who want to take responsibility for the logistics, and multiple government agencies would need to be involved.

Claims disputed

Neel died in 2000, while Chagnon, now almost 70 years old, is retired and living in Michigan.

However he did not respond to an Al Jazeera request for an interview related to the blood samples.

His supporters have in the past rejected claims that he acted improperly during his dealings with the Yanomami.

One supporter, who worked with Chagnon in field assignments in Brazil but did not want to be quoted, told Al Jazeera any suggestion he acted unethically was absurd and unfounded and that Chagnon had been made a scapegoat.

Ethical dilemma

The entire controversy has deeply divided many in the field of anthropology in the US, and forced people in the discipline to look at bigger issues.

"The Yanomami blood issue is ... a symbol of a deep problem in anthropology: Do anthropologists really care about the concerns of the people they study, or are they just interested in using the people to advance their own interests?" Robery Borofsky, an anthropologist at Hawaii Pacific University, told Al Jazeera.

Borofsky, who also wrote a book about the blood sample feud titled Yanomami: The Fierce Controversy and What we Can Learn About It," says that, in his opinion, the reasons the blood has not been returned is that is could be useful in future research, or that some organisations holding the blood are afraid of litigation.

"One can infer that they would like to keep the blood in case it might be used in some future human genome project. However, for the National Cancer Institute, and possibly Pennsylvania State University, it is a legal matter."

Borofsky says he has collected more than 4,000 signatures from anthropology students in the US and Canada asking for the blood to be returned and he will send those signatures to key individuals in Brazil and the US in February.

He also said he is considering publishing some of the letters in an advertisement in US and Brazilian newspapers.

"All the Yanomami want is their blood back," Borofsky said.

Everyone who has the blood says they want to return it, but this promise has remained unfulfilled, leaving many Yanomami leaders back in the villages of the Amazon left to wonder if ever it will be returned.

"I want that the white man listens to us, and respects our rights to have our blood back," said Kopenawa, the Yanomami leader.

"They took it, now it's up to them to return it. It has been over 40 years, and we do not want to wait anymore."